The present invention relates generally to positioning of mobile units by means of radio signals. More particularly the invention relates to a method for determining a position of a signal transmitter, a sensor and a system. The invention also relates to a computer program and a computer readable medium.
In modern life there are many situations in which an accurate positioning of a mobile unit is desirable, for instance in applications concerning freight, transportation, logistics and surveillance or when tracking soldiers and vehicles in a combat training area. In the latter case, the tracking data thus generated may either be used for later simulation or for online monitoring purposes. In any case, due to the normally harsh operating conditions, it is important that the tracking system be robust and provides a high degree of freedom of movement for the personnel and vehicles carrying the tracking units.
Today, there are a number of alternative solutions for accomplishing positioning data in respect of a mobile unit, which is associated with an individual or a vehicle. A GNSS (Global Navigation Satellite System, of any type) can potentially provide a high accuracy for outdoor applications, at least in the order of 10 cm. However, since the GNSS requires a line of sight to multiple satellites, the system tends to be highly inaccurate in urban environments. Moreover, the system suffers serious performance degradation in the presence of radio channel multipath components.
An urban system or an indoor system may instead be based on the received signal strength of so-called radio propagation fingerprints. A calibrated systems based on such fingerprints may attain an accuracy of 1 m. However, the system requires that radio channel and strength measurements be made before the system is established, and its performance is highly environment specific. Moreover, the system suffers from significant inaccuracies whenever the radio conditions are altered, for example as a result of pedestrian traffic or a change in the room arrangement.
Alternatively, a highly accurate system may be built on the basis of frequency references. This type of systems namely allows a very high accuracy in fixed line-of-sight conditions. However, such systems are very expensive, require oscillators of exceptionally high accuracy both at the transmitter and the receiver end, and do not operate well in mobile environments. Furthermore, the design suffers from performance degradation in non-line-of-sight conditions.
It is also technically possible to accomplish positioning in a cellular infrastructure, such as that of a mobile telephone system. However, here the accuracy is limited to 10 m, or worse.
The U.S. Pat. No. 5,179,573 describes a system which utilizes a digital correlator to determine the time of arrival of a received pseudonoise signal. Based on the time of arrival, in turn, a distance to a signal transmitter may be estimated. The correlation is performed by means of a low-resolution correlator with a low complexity, wherein an incoming phase code pattern is matched against an internally stored phase code pattern. N-bit in-phase and quadrature phase values are used to detect the time of arrival of the received signal. However, although the correlation process is iterative by involving one amplitude detection step and at least one time-of-arrival detection step, the correlation performed is exclusively of auto-correlation type, i.e. a matching between the incoming signal and local copies in the receiver representing the possibly transmitted sequences. Since this requires an extensive over-sampling, the complexity of the iterative process proposed in the U.S. Pat. No. 5,179,573 increases very rapidly for each correlation iteration. Thus, in practice, an exceptionally large amount of processing power is required to attain a level of positioning accuracy, which is generally desired in many of today's urban and indoor applications.